1. Field of the Invention
The present invention relates to a receiver and a gain control method thereof, and more particularly to a control scheme of a gain control amplifier provided in a radio section forming part of a receiver in mobile communication.
2. Description of the Related Art
Generally, in radio communication such as mobile communication, it is known that received power at an antenna terminal of a receiver largely varies due to the strength of radio waves depending on the distance between a transmitter and the receiver or due to the strength of radio waves depending on topographic conditions of a site where the transmitter communicates with the receiver. When the received power at the antenna terminal varies, there exists not only the need for setting a wide range of input power to a demodulator forming part of the receiver, but also the problem of failing to ensure stable reception characteristics due to the varying received power.
To address the problems, the receiver is provided with a gain control amplifier capable of controlling gain in a relatively later stage in a radio section forming part of the receiver and in a stage previous to the demodulator such that the gain of the gain control amplifier is controlled in accordance with variations in received power at the antenna terminal to maintain constant input power to the demodulator and to ensure stable reception characteristics.
Next, the control method of the gain control amplifier is described in detail. First, a received signal is demodulated in the receiver and the received power amount is derived from the demodulated received signal. Next, the derived received power amount is compared with a target value held in the receiver, and if the derived received power amount is lower than the target value, control is performed such that the gain of the gain control amplifier is increased. On the other hand, if the derived received power amount is higher than the target value, control is performed such that the gain of the gain control amplifier is reduced. In this manner, the input power amount to the demodulator is adjusted to be constant and stable reception characteristics can be realized.
FIG. 1 shows a configuration of the aforementioned receiver. The receiver shown in FIG. 1 comprises antenna 101 for receiving signals transmitted from a base station and for transmitting signals transmitted from a mobile station, antenna sharing device 102 for separating received signals and transmitted signals, high frequency amplifier 103 for amplifying a received signal in a radio frequency band, high pass filter 104 for passing only the frequency band of the received signal, frequency converter 105 for frequency converting the received signal in the radio frequency band into a received signal in an intermediate frequency band, channel filter 106 for passing only the signal of a received channel of the frequency converted received signal, gain control amplifier 107 capable of controlling gain, orthogonal demodulator 108, baseband filters 109 and 110, local signal oscillators 111 and 112, transmitter 113, and digital signal processing unit 114.
In the receiver configured as shown in FIG. 1, the received signal received at antenna 101 is inputted to high frequency amplifier 103 for amplification through antenna sharing device 102. The amplified output is inputted to frequency converter 105 through high pass filter 104. At frequency converter 105, the received signal in the radio frequency band is frequency converted into a received signal in the intermediate frequency band using a locally oscillated signal outputted from local signal oscillator 111, and then the converted signal is outputted.
The received signal in the intermediate frequency band outputted from frequency converter 105 is inputted to orthogonal demodulator 108 through gain control amplifier 107 after channel filter 106 filters out any interference wave existing near the received channel. At orthogonal demodulator 108, the received signal in the intermediate frequency band is frequency converted into a received signal in a baseband using a locally oscillated signal outputted from local signal oscillator 112, and the received signal is orthogonal demodulated for output as a received I component signal and a received Q component signal. Thereafter, the received I component signal and the received Q component signal are inputted to digital signal processing unit 114 through baseband filters 109 and 110, respectively.
Digital signal processing unit 114 converts analog signals into digital signals and performs digital signal processing such as error correction, and in addition, calculates the received power from the received signal to perform gain control of gain control amplifier 107.
Next, the control operation of gain control amplifier 107 is described in detail with reference to FIG. 2. It should be noted that FIG. 2 shows the details of the portions for processing the received signal in the intermediate frequency band and the portions for processing the received signal in the baseband (corresponding to digital processing unit 114 in FIG. 1) forming part of the aforementioned receiver, and the components equivalent to those in FIG. 1 are denoted with the same reference numerals. Since the portions of the receiver for processing the received signal in the high frequency band overlaps with the portions in FIG. 1, the description thereof is omitted.
Digital signal processing unit 114 comprises A/D converters 206 and 207 for receiving outputs from baseband filters 109 and 110 as their inputs, respectively, baseband digital filters 208 and 209, baseband signal processing unit 210, control signal generating unit 211, and power calculator 212.
The received I component signal and the received Q component signal outputted from orthogonal demodulator 108 are inputted to baseband filters 109 and 110, respectively, where any interference wave existing near the received signals is removed. Thereafter, the received I and Q signals are converted from analog signals into digital signals at A/D converters 206 and 207, respectively, and inputted to baseband digital filters 208 and 209. Baseband digital filters 208 and 209 respectively filter out any interference wave existing near the received channel and limit the bands for preventing intersymbol interference in the digital signals, and then the signals are inputted to baseband signal processing unit 210 and to power calculator 212, respectively.
Baseband signal processing unit 210 performs digital signal processing such as error correction. The interference wave existing near the received channel is sufficiently attenuated in three stages using channel filter 106, baseband filters 109 and 110, and baseband digital filters 208 and 209. Power calculator 212 calculates the received power amount from the received I and Q components signals and outputs the calculation result of the received power to control signal generating unit 211.
Control signal generating unit 211 compares the received power amount inputted from power calculator 212 with a target value held in control signal generating unit 211, and controls gain control amplifier 107 in accordance with the comparison result. Specifically, if the received power amount is lower than the target value, a control signal for increasing the gain of gain control amplifier 107 is generated to cause gain control amplifier 107 to increase its gain. Alternatively, if the received power amount is higher than the target value, a control signal for reducing the gain of gain control amplifier 107 is generated to cause gain control amplifier 107 to reduce its gain. The target value held in control signal generating unit 211 is a predetermined value so as to prevent saturation at input ends of A/D converters 206 and 207.
Next, a case is considered where the conventional receiver shown in FIG. 1 and FIG. 2 receives both a desired wave and an interference wave which exists with strong power to the desired wave in an adjacent channel to a received channel within a receiving frequency band. The desired wave and interference wave received at antenna 101 are inputted to channel filter 106 through high frequency amplifier 103, high pass filter 104, and frequency converter 105. In the conventional receiver shown in FIG. 1 and FIG. 2, since the interference wave existing near the received signal is attenuated in a total of three stages using channel filter 106, baseband filters 109 and 110, and baseband digital filters 208 and 209, the interference wave cannot be sufficiently attenuated only with channel filter 106. For this reason, the desired wave is inputted to gain control amplifier 107 with some interference wave remaining.
The received signal including the desired wave and some interference wave is orthogonal demodulated at orthogonal demodulator 108 after passing through gain control amplifier 107, and then baseband filters 109 and 110 again filter out the interference wave. However, the interference wave is not sufficiently removed, and the desired wave together with some remaining interference wave is inputted to A/D converters 206 and 207 for conversion from analog signal into digital signal, and then inputted to baseband digital filters 208 and 209 which sufficiently remove the interference wave of the received signal including the desired wave and interference wave. The signal is then inputted to power calculator 212 and to baseband signal processing unit 210.
Power calculator 212 calculates the received power amount of the received signal and outputs the calculation result to control signal generating unit 211 as described above. However, despite the reception of both desired wave and interference wave, the interference wave is sufficiently attenuated by channel filter 106, baseband filters 109 and 110, and baseband digital filters 208 and 209. Thus, the received power is not correctly calculated and control signal generating unit 211 is notified of the received power amount calculated only from the desired wave.
Control signal generating unit 211 compares the received power amount with the target value held in control signal generating unit 211 to generate a control signal for controlling the gain of gain control amplifier 107. If the received power amount of the received signal is lower than the target value, control signal generating unit 211 generates a control signal for increasing the gain of gain control amplifier 107 to increase the gain of gain control amplifier 107.
As described above, when the conventional receiver in a mobile unit shown in FIG. 1 and FIG. 2 receives both a desired wave and an interference wave which exists with strong power to the desired wave in an adjacent channel to a received channel, gain control amplifier 107 receives as its input the interference wave in addition to the desired wave and amplifiers both of them. However, the gain control signal for gain control amplifier 107 is a control signal obtained after the interference wave has been sufficiently removed. Therefor, the interference wave causes saturation at the input ends of A/D converters 206 and 207 to result in disadvantages that favorable reception characteristics can not be obtained and the bit error rate is increased in digital communication.
When reference is made to the technology in JP-A-10-126301 proposed by the present inventor, it discloses the invention for controlling the gain of a receiver with emphasis on a characteristic as found that an nth order (nxe2x89xa72) distortion component caused by nonlinearity of a high frequency amplifier or frequency mixer forming part of the receiver is increased or reduced by na (dB) in proportion to an increase or reduction in electrical field intensity by a (dB).
Specifically, a variable gain amplifier is provided at a front end of the receiver and, after the reception of a transmitted signal at the receiver, if attenuation of the gain amount of the variable gain amplifier by a (dB) resulting from control of the gain amount causes the electrical field level of the received signal to be attenuated by b (dB) or smaller (a less than b), the control of the gain amount of the variable gain amplifier is not performed since it is considered that no cross modulation from the interference wave occurs. When attenuation of the gain amount by a (dB) causes the electrical field level of the received signal to be attenuated by b (dB) or larger, the gain amount of the variable gain amplifier is further controlled since it is considered that cross modulation occurs from a plurality of interference waves, such that the gain amount is increased up to a value at which no influence due to cross modulation is found in the bit error rate of the received signal.
The aforementioned technology in JP-A-10-126301 assumes a cross modulation wave as an interference wave of interest, but does not contemplates the aforementioned case where a receiver receives both desired wave and interference wave which exists with strong power to the desired wave in an adjacent channel to a received channel.
It is an object of the present invention to provide a receiver and a gain control method thereof which, even when a received signal includes not only a desired wave but also an interference wave with strong power adjacent thereto, can prevent saturation at input ends of an A/D converters caused by the interference wave to maintain favorable reception characteristics and to suppress an increase in bit error rate in digital communication.
According to the present invention, a receiver is provided. The receiver comprising selecting means for selectively deriving a desired wave in a received wave, gain control amplifying means for amplifying an output from the selecting means, demodulating means for demodulating the amplified output, and gain control means for comparing a power amount corresponding to the demodulated output with selected and set one of a plurality of target values to control gain of the gain control amplifying means in accordance with the comparison result, wherein the gain control means includes target value switching control means for switching the one target value to another target value in accordance with the presence or absence of an interference wave contained in the demodulated output.
In addition, the receiver further comprises digital signal processing means for digitally processing a baseband signal which is the demodulated output from the demodulating means. The digital signal processing means includes a digital converter for digitizing the baseband signal and a filter for filtering out the interference wave component from the digital signal. The target value switching control means includes interference wave detecting means for detecting the presence or absence of the interference wave based on an output power amount from the filter and a saturated operation state of the digital converter.
The target values includes a predetermined first target value and a predetermined second target value which is lower than the first target value. The target value switching control means sets the first target value at an initial state, and switches to the second target value when the interference wave detecting means detects the interference wave in that state. The first target value is predetermined such that a received power amount at an input terminal of the digital converter causes no saturated operation of the converter in a normal receiving state with no interference wave present.
The receiver further comprises digital signal processing means for digitally processing a baseband signal which is the demodulated output from the demodulating means. The digital signal processing means includes a digital converter for digitizing the baseband signal and a filter for filtering out the interference wave components from the digital signal. The target value switching control means includes interference wave detecting means for detecting the presence or absence of the interference wave based on an output power amount from the filter and a state of the digital signal.
The interference wave detecting means detects an error rate as an output state of the digital signal, and detects the presence or absence of the interference wave based on the error rate. The interference wave detecting means detects the presence of the interference wave when the error rate is equal to or higher than a predetermined threshold value.
The target value switching control means switches the target value from an initial set value to a second target value which is lower than the initial set value by a certain amount when the error rate is equal to or higher than the threshold value, and further switches the target value from the second target value to a third target value which is lower than the second target value by a certain amount when the error rate is equal to or higher than the threshold value. The target value switching control means stops switching of the target value only when the error rate becomes lower than the threshold value. The target value switching control means switches the target value to the initial set value after a certain time has elapsed after the stop of switching of the target value.
According to the present invention, a method of controlling gain in a receiver is provided. The method comprises the steps of selectively deriving a desired wave in a received wave, amplifying the selected output, demodulating the amplified output, digitally processing and outputting the demodulated output, comparing a power amount corresponding to the demodulated output with selected one of a plurality of target values for a power amount, amplifying the selected output in accordance with the comparison result, and controlling switching of the selected one target value in accordance with the presence or absence of an interference wave contained in the demodulated output.
The receiver is designed to digitize a baseband signal which is the demodulated output with a digital converter to filter out the interference wave component from the digital signal with a filter. The target value switching control step includes an interference wave detecting step for detecting the presence or absence of the interference wave based on an output power amount from the filter and a saturated operation state of the digital converter.
The interference wave detecting step detects the saturated operation state when the digital converter provides a continuous maximum output. The target values includes a predetermined first target value and a predetermined second target value which is lower than the first target value. The target value switching control step sets the first target value at an initial state, and switches to the second target value when the interference wave detecting step detects the interference wave in that state. The first target value is predetermined such that a received power amount at an input terminal of the digital converter causes no saturated operation of the converter in a normal receiving state with no interference wave present.
In addition, the receiver is designed to digitize a baseband signal which is the demodulated output with a digital converter to filter out the interference wave components from the digital signal with a filter. The target value switching control step includes interference wave detecting step for detecting the presence or absence of the interference wave based on an output power amount from the filter and an output state of the digital signal.
The interference wave detecting step detects an error rate as the output state of the digital signal, and detects the presence or absence of the interference wave based on the error rate such that it detects the presence of the interference wave when the error rate is equal to or higher than a predetermined threshold value. The target value switching control step switches the target value from an initial set value to a second target value which is lower than the initial set value by a certain amount when the error rate is equal to or higher than the threshold value, and further switches the target value from the second target value to a third target value which is lower than the second target value by a certain amount when the error rate is equal to or higher than the threshold value. The target value switching control step stops switching of the target value only when the error rate becomes lower than the threshold value.
As described above, although a single target value is conventionally set for generating a gain control signal, in the present invention, a plurality of target values are set which can be switched at a baseband signal processing unit. Specifically, the baseband signal processing unit monitors the power amount of the demodulated baseband signal and the state of the digital signal, and switches the initial set target value to the second target value which is lower than the initial value in response to a continuous high digital signal, a deteriorated bit error rate or the like. The target value is compared with the received power amount to control the gain of the gain control amplifier.
When a deteriorated state of the demodulated digital signal causes a deteriorated bit error rate or the like, A/D converters are at a saturated state since the input power at the inputs ends of the A/D converters is increased. Thus, it can be considered that any interference wave is included in the received signal. In this case, control for reducing the target value is performed to accordingly reduce the power at the input ends of the A/D converters, thereby achieving favorable characteristics such as the bit error rate of the demodulated digital signal or the like.
The above objects, features, and advantages of the present invention will become apparent from the following description based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.